1. Field of the Invention
The present invention relates to a motor driving apparatus, and more particularly to a motor driving apparatus that stores regenerative energy recovered during motor deceleration into a capacitor and that uses the recovered energy as powering energy during motor acceleration.
2. Description of the Related Art
[Background Art]
FIG. 13 is a circuit diagram of a motor driving apparatus according to the prior art. The motor driving apparatus 101 shown in FIG. 13 includes a converter 2, an inverter 3, a motor 4, an inverter control circuit 5, and a smoothing capacitor 6. In FIG. 13, reference numeral 109 indicates a converter output current, 110 a converter input current, 111 a DC link current, 112 a capacitor output current, and 113 a DC link voltage (capacitor voltage). For reference numerals 110 to 113, a description will be given later with reference to FIGS. 14(A) to 14(F) by using the results of simulation of waveforms in operation of the motor driving apparatus 101.
AC power is supplied to the converter 2, which rectifies the AC to DC and charges the smoothing capacitor 6. The inverter 3, which converts the DC to AC, receives the DC voltage energy stored in the capacitor 6, and drives the motor 4, a three-phase induction motor, by switching the transistors forming the inverter 3 on and off under the control of the inverter control circuit 5.
FIGS. 14(A) to 14(C) are diagrams showing the relationship between motor speed and power in the motor driving apparatus according to the prior art, and FIGS. 14(D) to 14(F) are time charts showing the results of simulation of the various parts during operation. In FIGS. 14(A) to 14(F), the abscissa represents the time, the ordinate in FIG. 14(A) represents the motor speed RPM, the ordinate in FIG. 14(B) represents the power kW supplied to the motor, the ordinate in FIG. 14(C) represents the power kW supplied from the power supply to the motor, the ordinate in FIG. 14(D) represents the DC link current, the ordinate in FIG. 14(E) represents the converter input current and the capacitor output current, and the ordinate in FIG. 14(F) represents the DC link voltage (capacitor voltage).
As can be seen from FIGS. 14(A) to 14(C), power is supplied from the capacitor C6 to the motor 4 during a prescribed period from time t11 to time t12 in the early part of acceleration, and power is supplied to the motor 4 from the power supply as well as from the capacitor C6 during a prescribed period from time t12 to time t13 in the late part of acceleration. That is, the energy stored in the capacitor C6 is supplied to the motor 4 during the first half period of acceleration, starting from acceleration start time t11 to time t12, and not only the energy stored in the capacitor C6 but also the power from the power supply is supplied to the motor 4 during the second half period of acceleration from time t12 to time 13.
If the output required of the motor 4 is large during the second half period of acceleration from time t12 to time 13, the energy stored in the capacitor C6 may be used up, in which case only the current from the power supply is supplied to the motor 4. As a result, the peak of the input current increases. As described above, the regenerative energy stored in the capacitor C6 is supplied to the motor 4, starting from the acceleration start time t11.
During the constant speed period from time t13 to time t14 after the end of the acceleration, power is supplied from the power supply to the motor 4, and during the deceleration period from time t14 to time t15, regenerative energy is stored in the capacitor C6.
Patent document 1 discloses a motor driving apparatus in which a capacitor for storing regenerative energy is connected in parallel between a converter, which receives AC voltage and converts it to DC power, and an inverter (power driver), which receives the DC power and converts it to AC power, wherein the regenerative energy recovered during motor deceleration is stored in the capacitor, and the thus stored energy is used as powering energy during motor acceleration. According to this motor driving apparatus, by using the regenerative energy as powering energy, the power to be supplied from the power supply decreases, and the average power thus decreases. In this motor driving apparatus, the regenerative energy stored in the capacitor is supplied to drive the motor, starting from the time that the motor begins to accelerate.
FIGS. 15(A) to 15(D) are diagrams showing motor output in the motor driving apparatus according to the prior art. In FIGS. 15(A) to 15(D), FIG. 15(A) shows the output of the main motor (indicated by reference numeral 4 in FIG. 13), FIG. 15(B) shows the power supplied to the main motor 4, FIG. 15(C) shows the output of a sub-motor, and FIG. 15(D) shows the power supplied from the power supply to the main motor 4. In FIGS. 15(A) to 15(D), the abscissa represents the time t, and the ordinate represents the output kW.
As shown in FIG. 15(A), when power is supplied to the main motor 4, the output of the main motor 4 begins to increase at output start time t0 and continues to increase gradually until time t10 at which the motor reaches a constant speed; then, the output begins to decrease at output end time t22 and continues to decrease gradually until time t30 at which the motor stops.
As shown in FIG. 15(B), since there is a limit to the power supplied from the capacitor (indicated by reference numeral 6 in FIG. 13) to the main motor 4, the capacitor 6 may cease to supply power to the main motor 4 in the middle of the motor operation. FIG. 15(C) shows the output of the sub-motor not shown in FIG. 13, and FIG. 15(D) shows the power supplied from the power supply to the main motor 4.
Since the power being supplied from the capacitor 6 to the main motor 4 may cease, as described above, power is supplied from the power supply to the main motor 4 during the period from time t20 to time t30 after the capacitor 6 has ceased to supply power to the main motor 4. If it is required that power be supplied to the sub-motor (not shown in FIG. 13) after time t20, i.e., after the capacitor 6 has ceased to supply power, power cannot be supplied from the capacitor 6 to the main motor 4 after that time t20. As a result, after time t20, power is supplied to the main motor 4 only from the power supply.
As shown in FIG. 15(D), during the period from time t2 to time t20, the power to the main motor 4 is supplied from the power supply as well as from the capacitor 6, and during the period from time t0 to time t2 and the period after time t20, the power is supplied only from the power supply.
As can be seen from the above, in the motor driving apparatus according to the prior art, the electric energy stored in the capacitor 6 cannot be used where the supply of power to the main motor 4 is needed in order to meet the requirement of the machine, but is used where it is not needed, and hence the problem that the electric energy stored in the capacitor 6 is not used efficiently in the motor driving apparatus.
[Patent document 1] Japanese Unexamined Patent Publication No. 2000-141440 (refer to the claim in [claim 1], the description from paragraph [0014] to paragraph [0025], and the drawings [FIG. 2] and [FIG. 3])